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GeneralTo star or not to star...

29th Dec 2019 17:38 UTCMatt Courville

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Hello, I had stumbled recently upon the 'Star of Adam'  Blue Star Sapphire as posted and was thinking to myself as a field collector and lapidary enthusiast about the staring effect of corundum in general.   Perhaps someone might be able to clarify some thoughts...    Say one were to collected many dozen corundum crystals of any color and wished to know whether or not to trim, cut and polish these into what could be 'star-ing effects' what would determine choosing one over the other?

Should the rough corundum already have an effect present, or is it trial and error.  When looking at the photo which I had posted I see a beautiful polished highly valuable piece that could have possibly looked like a non-transparent, non-translucent, blue-ish chunk of what could have been a poorly terminated crystal; and therefore seemingly a large shelf piece. 

Any thoughts, as I would love to wrap my head around the decision making process of what is 'worthy'  ;)

Matt


29th Dec 2019 17:54 UTCPeter Slootweg 🌟

In my experience this effect is usually already visible in the rough material. Not the complete star, but a satiny sheen perpendiculair to the c-axis. The star will only appear when cut as a cabuchon. Check the flat underside of a star sapphire and you know what i mean.

29th Dec 2019 18:04 UTCcascaillou

polish a tiny window perpendicular to the optical axis, add a drop of oil, and try and see if some asterism appears under strong directional light.

Beware of fakes (asterism in corundum can be intensified or created from heat treatment or titanium-diffusion treatment, there is also asteriated synthetic corundum, you might also encounter cabbed doublet stones with asterism, etc...not to mention that a stone with a 100% natural star could still have been treated just to improve its color and/or clarity)


29th Dec 2019 18:09 UTCKevin Conroy Manager

A good starting point would be to find out if "stars" are found where you're collecting.   Build off of someone else's groundwork.

29th Dec 2019 18:45 UTCMatt Courville

great info thanks - I never thought about the staring effect being localized
 
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